Radio frequency (RF) communication involves the transmission of electrical energy through free space in the form of electromagnetic radio waves. Information carried by the radio waves may be controlled using various analog signal encoding techniques, the most common of which are amplitude modulation (AM) and frequency modulation (FM). Emerging satellite radio (XM) transmits radio waves at high frequencies via digital signal modulation.
Multiple receiving antennas may be used in what is referred to as an antenna diversity scheme. Antenna diversity may improve signal reception where multiple receiving paths exist, or in instances where reception by a single antenna is somehow obstructed. Conventional antenna diversity schemes require signal detection and/or demodulation, as well as complex decision algorithms to select between multiple antenna input signals. Since the decision algorithms and antenna control steps occur at a receiver, an RF cable is typically required between each diversity antenna and the receiver. The complexity of the diversity receiver and the required multiple RF cables often renders the use of a conventional antenna diversity scheme too costly for use in some systems, such as aboard a vehicle or in other multi-path RF applications.